The number of serious "warning letters" sent by the Food and Drug Administration to drug, device and food companies has dropped by half over the past five years, leading to a situation where the agency's ability to ensure that products are being made properly has been "significantly compromised," according to a study by the Democratic staff of the House Government Reform Committee.

In addition to a drop of almost 50 percent in the number of on-site inspections at manufacturing plants, the 15-month probe found that in some cases, FDA headquarters rejected the enforcement recommendations of agency field offices despite inspectors' findings that violations had led to deaths or serious injuries.

In 32 cases, investigators found, field inspectors recommended that the FDA issue a warning letter, seize the product or seek a court injunction, but the agency sent a significantly weaker letter alerting the company to possible problems.

"Under the Bush administration, enforcement efforts have plummeted and serious violations are ignored," said Rep. Henry A. Waxman (D-Calif.), who requested the study. "FDA can't do its job when its enforcement arm is tied behind its back."

Office of Enforcement Director David Elder responded that regulators have been focusing on situations that pose the greatest risk to consumers and the public health.

"FDA enforcement cannot be properly judged by counting the number of actions taken by the agency," he said in a statement. "FDA has increasingly used an enforcement strategy based on efficient risk management principles that focus on combating the greatest public health risks and maximizing our deterrent effect against potential violators."

In addition, Elder said, the agency has won unprecedented legal actions against firms found to be in violation of the law. Fines and penalties from those major prosecutions and settlements have brought more than $2.5 billion since 2000, he said.

According to the committee's minority report, the number of warning letters declined from 1,154 in 2000 to 535 in 2005. The biggest-percentage decline occurred in the agency's Center for Devices and Radiological Health, which had 66 percent fewer warning letters issued in 2005 than in 2000. That center regulates defibrillators and a number of other devices that have been the subject of recalls and controversy over defective and failed products. At the FDA's Center for Drug Evaluation and Research, the number of warning letters dropped 39 percent.

The inspections, warning letters and enforcement actions described in the report involve questions of whether manufacturers are following good manufacturing practices -- whether their pills, medical devices and food products are being produced in a reliably safe manner.

Over the past decade, the agency has to some extent moved from on-site inspections to assure quality toward trying to make sure that companies have proper manufacturing and quality-control systems.

The minority staff's report concludes that the number of warning letters has not been halved because the plants are safer and better run. They found that FDA inspectors wrote about as many reports describing significant violations in 2000 as in 2005, a sign that problems remained. Those violation reports, however, were less likely to result in warning letters or other enforcement activities.

Waxman asked for the study because of reports that FDA inspectors who identified problems at Chiron Corp.'s flu-manufacturing plant in England were overruled by their supervisors. British drug authorities closed the plant because of the manufacturing problems, leaving the United States with a major shortage of flu vaccine for the winter of 2004-2005.